Technical Field
The present invention relates to a method for operating a wind power installation. The present invention furthermore relates to a wind power installation. The present invention furthermore relates to a state observer for observing an available power of a wind power installation.
Description of the Related Art
Wind power installations and methods for operating them are known. Wind power installations feed electrical power into an electrical supply network, and are also being used increasingly to support the electrical supply network electrotechnically, where appropriate.
One possibility of such support is that, after a frequency dip, the wind power installation can temporarily feed a higher power into the electrical supply network, which may for simplicity also be referred to merely as the network, which is higher than the wind power installation can generate on the basis of the prevailing wind conditions at that time. Specifically, in addition to the power which is generated from the wind, electrical power from the kinetic energy of the rotor is also used therefor. This involves, in particular, the aerodynamic rotor of the wind power installation as well as an electromagnetic rotor, coupled thereto, of the generator. This applies in particular for gearless wind power installations in which the rotor of the generator is coupled directly to the aerodynamic rotor and the rotor also has a significant moment of inertia. This kinetic energy of the rotor affects all these rotating parts.
In any event, the feeding of this kinetic energy leads to the speed of the rotor being reduced. After this supporting measure is over, or can be ended, the wind power installation should return to its normal operating state. In simple terms, for the assumption of constant wind speed over the period of time considered, this means that the rotational speed must be increased again, and the electrical power of the generator must to this end necessarily lie below the driving power generated by the aerodynamic rotor for a certain period of time, and can therefore only be increased again slowly.
Such a rotational speed increase, i.e., returning the installation to the normal operating state, may however entail problems. Firstly, there is always the risk that the wind power installation, in this low rotational speed operating state which has occurred unavoidably, may also be in an aerodynamically less effective operating state. The installation must correspondingly be operated carefully in this state, which also applies for returning to the normal operating state. Furthermore, the reduction of the electrical power for the purpose of reacceleration may under certain circumstances have detrimental effects on the network, which after the previous disturbance still relies on a maximally constant and high power input from the wind power installation.
In the priority application for the present application, the German Patent and Trademark Office has searched the following prior art documents: DE 10 2009 014 012 A1, DE 11 2005 000 681 T5 and WO 2011/124696 A2.